Many systems, such as industrial or commercial controls, thermostats, appliances, factory automation products or heating, ventilating or air conditioning (HVAC) systems are configured, tested or monitored by monitoring of electrical properties. Today modern protection, control, and monitoring (PCM) systems consist of microprocessor-based multifunction relays.
Multifunction relays have built-in diagnostics that alarm for an internal failure, an important feature of the relay. However, the relay cannot test for a failure or misapplication of input or output. Misapplying an output contact can result in a breaker failure to trip, a failure to reclose, or a failure to perform a needed control action. Self-tests also miss human errors, such as applying the wrong voltage for digital input or connecting DC power outside the rating of the relay. Misapplying user programming functions can also cause misoperation. The following are some examples of these errors:                Failure to program a trip output contact to assert when the relay issues a trip command;        Wrong relay element programmed in the trip equation;        Wrong relay elements programmed in a pilot blocking scheme; or        Incorrect logic settings disable sensitive instantaneous overcurrent element for a hot-line condition;        Brownout conditions;        Incorrect directional element impedance setting in pilot blocking scheme; or        Incorrect phase rotation.        
A particular example can be found a common home appliance, a washing machine. Modern washing machines are simple for users to operate, but engage in a complex array of device state controls to operate. Subsystems in washing machines must engage/disengage hot water input and cold water input. A motor must be controlled to agitate or spin. Draining must be enabled or disabled. Spinning may be faster or slower. A washer may have a heating element to heat or maintain a temperature of water used in the wash process. A washer must control when soap, fabric softener or bleach are added to a washing process. Mechanical motion is monitored. Alarms are generated for device malfunctions or status of a washing operation. Functions such as these are controlled with switches, relays, valves, transducers, sensors and user interfaces. Miswiring of electrical connections can result in device failure, or even injury.
Testing device configurations, such before a device leaves the factory, during device operation or after servicing, can be done by monitoring electrical characteristics. Such characteristics may check for short circuits or open circuits, the results of which can be compared to expected results for a particular device state. Voltage levels can also be monitored and compared with expected values.